1. Field of the Invention
The present invention relates to an optical transmitting and receiving system, and more particularly to a signal superimposing method in optical communications. The optical transmitting and receiving system and the signal superimposing method are also effective in quantum communications.
2. Background Art
Secrecy requirement in communication has been and will be studied, but might never be fully satisfied. In the recent networking society, advances in cryptography have reasonably fulfilled such requirement. For example, the public key cryptosystem, which is widely used at present, is a software-based cryptograph based on the fact that it takes unrealistically long time to break a cryptogram even when a high-speed computer is used. On the other hand, quantum cryptography, which has been actively studied in recent years and uses a principle based on physical laws, may pave the way for a new hardware-supported cryptosystem.
Although different from quantum cryptography in terms of purpose, there is a hardware-oriented approach that attempts to make optical communications themselves secure. At present, optical fibers are generally used in communications as a transmission channel in intermediate and long-haul transmission and output light from a laser diode (LD) is used as signal light. The output light from an LD is well described by the state called a coherent state, and the magnitudes of fluctuations of two quadrature components are equal to each other, which are equal to the magnitude of vacuum fluctuations. The fluctuations of an electromagnetic field can be controlled, and the so-called squeezed state is obtained by reducing the magnitude of fluctuations of one of the quadrature components and increasing that of the other. In the study described in T. Tomaru and M. Ban, Phys. Rev. A 74, 032312 (2006), signals are masked by increased fluctuations (antisqueezed component) for secure communication. The term “squeezed state” refers to the state in which the magnitude of the reduced (squeezed) fluctuations is smaller than that of the vacuum fluctuations. In a method using antisqueezing, it does not much matter whether or not the magnitude of fluctuations of the squeezed component is smaller than that of the vacuum fluctuations. The magnitude of fluctuations of the squeezed component can be larger than that of the vacuum fluctuations as long as the magnitude of fluctuations of the antisqueezed component is sufficiently large. In general, light with the magnitude of one of fluctuation components being larger than that of the other in a phase space is called antisqueezed light. That is, squeezed light is one type of antisqueezed light (T. Tomaru and M. Ban, Phys. Rev. A 74, 032312 (2006)). Antisqueezed light can be produced, for example, using the Kerr effect of an optical fiber (JP Patent Publication (Kokai) No. 2006-191410).